EVIDENCE FOR VIP-INDUCED INCREASE IN NO PRODUCTION IN MYENTERIC NEURONS OF OPOSSUM INTERNAL ANAL-SPHINCTER

A significant interaction between vasoactive intestinal polypeptide (V IP) and nitric oxide (NO) has been reported in neurotransmission of th e gastrointestinal tract, including the internal anal sphincter (IAS). The exact site of this NO release from the IAS in response to VIP is not known. Studies were carried out to determine the site of VIP-induc ed NO release in opossum IAS. NO synthase (NOS) activity was quantitat ed by determining L-[H-3]citrulline production from L-[H-3]arginine in isolated myenteric ganglia and smooth muscle cells of the IAS. L-[H-3 ]citrulline production was determined before and after treatment with either the ganglionic stimulant 1,1-dimethyl-4-phenylpiperazinium iodi de (DMPP), VIP, or peptide histidine isoleucine (PHI) in the absence a nd presence of the neurotoxin tetrodotoxin and the NOS inhibitor N-G-n itro-L-arginine (L-NNA). Smooth muscle cells and ganglia were preloade d with L-[H-3] arginine for 5 min and treated with VIP for 1 and 5 min . DMPP and VIP caused a significant increase in L-[H-3]citrulline prod uction in the myenteric gan glia, whereas PHI had no effect. VIP cause d a significance increase in L-[H-3]citrulline formation in myenteric ganglia at both time periods, whereas in smooth muscle cells there was a moderate but significant increase in L-[H-3]citrulline formation on ly at 5 min of VIP treatment. VIP-induced relaxation of isolated smoot h muscle cells of the IAS was not affected by L-NNA. The increase in N S activity of myenteric ganglia by DMPP and VIP was sensitive to neuro toxin and the NOS inhibitor. The data suggest that the increase in NO production in response to VIP in the LAS occurs mainly from the myente ric neurons, with some contribution from the smooth muscle cells.